1. Field of the Invention
The present invention relates to an image forming apparatus represented by a printer and a copier.
2. Description of the Related Art
Among image forming apparatuses, ink-jet printers have widely spread as home printers in recent years. An ink-jet printer is an image forming apparatus in which ink is ejected from a multitude of nozzles provided in a recording head as a recording device to print a sheet or the like as a recording medium. Ink-jet printers have become popular because of their advantages of small size, inexpensiveness and good quietness, and manufacturers provide a multitude of models nowadays. In recent years, there have been developed ink-jet printers adapted to office use, which include a recording head (line head) having substantially the same length as the width of sheets normal to a sheet conveying direction and are capable of high-speed printing of several tens of copies per minute, although the number of available models is still small.
Japanese Unexamined Patent Publication No. 2006-206309 discloses an example of the ink-jet printer as described above. The ink-jet printer includes a recording medium tray (sheet cassette), an image recording unit (recording device), a platen unit (conveyance unit) as a belt conveyor device and a discharge tray.
A plurality of recording media such as cut paper, OHP sheets or other sheets are accommodated while being stacked one above another on the recording medium tray and fed to the platen unit by a feeder. The platen unit includes an endless conveyor belt for conveying sheets, and a sheet fed from the recording medium tray is placed on the upper surface of this conveyor belt to be substantially horizontally conveyed.
The image recording unit is arranged above the platen unit to face it and includes inkjet head arrays for ejecting ink to the sheet corresponding to a plurality of colors of inks such as black, cyan, magenta and yellow. The ink head arrays extend in a main scanning direction, i.e. in a sheet width direction normal to a sheet conveying direction. Each inkjet head array ejects the ink in a gravitational direction, i.e. in a vertically downward direction from a multitude of nozzles.
The sheet being conveyed on the platen unit is printed with the inks ejected from the inkjet head arrays of the image recording unit arranged above in its process of conveyance. A plurality of colors of inks is successively ejected to the sheet to form a color image. The completely printed sheet is conveyed to the discharge tray as it is to be discharged.
Here, in the ink-jet printer constructed as above, nozzles are generally capped to prevent the drying and clogging of the ink ejection nozzles of the image recording unit. The ink-jet printer is further provided with an elevator for moving the platen unit (conveyance unit) upward and downward in order to cap the nozzles and deal with a jam having occurred during the conveyance of the sheet.
The apparatus of Japanese Unexamined Patent Publication No. 2006-206309 is provided with a platen driver as the elevator for moving the platen unit upward and downward. This platen driver includes four arms extending from a supporting shaft, and moves the platen unit upward and downward by rotating the arms while supporting the bottom surface of the platen unit with the tips of the arms.
However, since the relatively long arms extend from the supporting shaft in such an elevator for the platen unit, excessive torques might act on the arms themselves, the supporting shaft and a driving device such as a motor. This might possibly cause the arms and the motor to be deformed or broken. Therefore, problems of reducing image quality and shortening the life of the apparatus occur.